Field of the Invention
The present invention relates to a method for producing a hot dip plated steel sheet and an apparatus for hot dip plating used in the method.
The present application is a national stage application of International Application No. PCT/JP2009/005089, filed Oct. 1, 2009, which claims priority to Japanese Patent Application No. 2008-256208, filed on Oct. 1, 2008, the content of which is incorporated herein by reference in its entirety.
Description of Related Art
In a process of producing a hot dip plated steel sheet, a coating weight is controlled by injecting a gas from a wiping nozzle toward a steel sheet during a time when the moving steel sheet (steel strip) is continuously immersed into a plating bath, the steel sheet is pulled up from the plating bath, and then plating metal coated onto a surface of the steel sheet is solidified. At this time, oxide films (dross) are formed on the plated surface of the steel sheet due to the oxidization of the molten plating metal, which degrades the appearance of a product.
In order to prevent the oxidization of the plating metal, there is proposed a technique in which an entire region from a bath surface of the plating bath to a gas injection position of the wiping nozzle is covered with a seal box and an inert gas is introduced into the seal box so as to decrease the oxygen concentration of the entire atmosphere in the seal box (for example, see Japanese Unexamined Patent Application, First Publication No. H11-140615, No. S62-30864, No. H04-285148, and Japanese Examined Patent Application, Second Publication No. S61-34504). According to this technique, since it is possible to decrease the oxygen concentration to less than that of an ambient atmosphere during a time when the steel sheet is pulled up from the plating bath and the molten plating metal is solidified, it is possible to prevent the oxidization of the plating metal.
However, in the technique of covering the entire region from the bath surface of the plating bath to the gas injection position of the wiping nozzle by using the seal box as in the techniques disclosed in Japanese Unexamined Patent Application, First Publication No. H11-140615, No. S62-30864, No. H04-285148, and Japanese Examined Patent Application, Second Publication No. S61-34504, it is possible to obtain the advantage of suppressing the formation of the oxide films, but it is difficult to visually recognize the gas injection position for controlling the coating weight which is important for the hot dip plating operation. In addition, it is difficult to remove surface oxide films formed on the surface of the plating bath or to maintain the wiping nozzle. For this reason, there is a problem in that the operation is inconvenient. In addition, in the case where a surface of a plating liquid is not covered with a certain amount of oxide films, zinc fume is generated from the surface. When metallic zinc is adhered to an apparatus such as the wiping nozzle due to the zinc fume, it is not possible to normally perform the wiping operation. For this reason, there is a problem in that the quality of the product is degraded. Accordingly, when the techniques are used in a practical application, there is a problem in that the operability and the plating quality are degraded.
The present invention is contrived in consideration of such circumstances, and an object of the present invention is to provide a method for producing a hot dip plated steel sheet and an apparatus for hot dip plating used in the method capable of suppressing the formation of the oxide films on the surface of the plated steel sheet during controlling the coating weight and eliminating the disadvantages in the operation and quality.